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So now that legendaries can get regional forms, who do you think will be next?
I've given this a lot of thought lately and I think the birds are probably the only trio that makes the most sense to get forms because they're the only legendary trio where it's hard implied there are multiples of their species and they're just really strong regular animals basically. All of the other trios either have lore reasons where it where it wouldn't make sense for them to be anything but what they are or you can do something better with them (like just make more Regis, which we got). Box legendaries similarly seem to be locked by their lore so it would be kind of weird, especially because so many of them now already have alternate forms.
I think the only real options would be mythicals or the legendary beasts. Beyond that it would seem like they were grasping at straws to come up with crap like Alolan Cobalion. Also, a lot of the legendaries it just makes sense to give them stronger forms if they don't have them already.